Artificial intervertebral disc

ABSTRACT

An artificial intervertebral disc for at least partially replacing a diseased or damaged intervertebral disc. The artificial disc includes a concave-convex articulating surface. The artificial disc can be used in the cervical region of the spine, where a concave-convex articulating surface is advantageous for improved anatomical fit and region appropriate kinematics. The artificial disc of the present invention also includes an anchor for attachment to bone.

FIELD OF THE INVENTION

The present invention generally relates to an apparatus for thetreatment of spinal disorders and, in particular, an artificialintervertebral disc, an implant that replaces a diseased or damagedintervertebral disc.

BACKGROUND OF THE INVENTION

There are many painful disorders of the spine, many relating, at leastin part, to diseased or damaged intervertebral discs. Disorders includeDegenerative Disc Disease, generally an age related disorder where theintervertebral disc gradually loses its water content, resiliency, andheight. With a loss in intervertebral disc height and associated loss ofnormal spacing between vertebrae, motion of the vertebrae can placepressure on the spinal cord or exiting nerve roots. The intervertebraldisc itself can also be a source of pain. Spinal disorders, commonlyreferred to as disc herniation and bulging disc, place painful pressureon the spinal cord and exiting nerve roots. Abnormal bone growth, calledosteophytes, can place pressure on nerves or the spinal cord. Often, asurgeon must at least partially remove an intervertebral disc to accessand remove an osteophyte.

A surgical approach to treating chronic spinal disorders relates to bonyfusion of two adjacent vertebrae in a treatment called spine fusion.Following the achievement of appropriate spacing and alignment of thevertebral bodies, bone graft material and stabilization provide anenvironment for spine fusion. Implant systems, to include plate and rodsystems and interbody devices, such as, interbody spacers and fusioncages can be used to support the spine during fusion. Concerns persistregarding spinal fusion treatment stemming from modest clinical successrates and the creation of rigid regions along an otherwise flexiblespine.

Artificial intervertebral discs, or simply artificial discs, are analternative to spinal fusion and represent an emerging technology. Thesespinal implants are designed to restore or maintain the appropriatealignment and spacing of adjacent vertebral bodies. In addition, anartificial disc is also designed for kinematic behavior similar to ahealthy natural disc. Known artificial disc concepts use numerous meansfor providing motion and stiffness similar to a natural healthy disc, toinclude the adaptation of elastomers, mechanical springs, andarticulating surfaces.

Prior art artificial discs often use articulating surfaces to create ajoint between adjacent vertebrae. Disc implants using articulatingsurfaces rely on methodology and proven technology used in total jointarthroplasty of the hip, knee, and shoulder. Numerous prior artartificial discs resemble artificial hip and artificial knee joints.Numerous known artificial disc devices resemble variations of aball-and-socket. Kuntz, in U.S. Pat. No. 4,349,921 (Sep. 21, 1982)discloses an artificial disc, with two components that articulate bymeans of a projection on one component pivotally engaging a depressionon the second component. An artificial disc resembling an artificialknee joint has also been suggested. Shelokov, in U.S. Pat. No. 6,039,763(Mar. 21, 2000) discloses an artificial spinal disc, similar inconfiguration to an artificial knee joint.

Heggeness et. al., in U.S. Pat. No. 5,514,180 (May 1996) categorizes theshape or contours of vertebral endplates into five groups: “ramp”,“saddle”, “irregular”, “bowl”, and “hump”. Heggeness et. al., teachesthe importance of endplate shape relating to fit and load distributionof a prosthetic devices within intervertebral disc spaces, but Heggenesset. al. does not discuss endplate shape relating to articulatingsurfaces or spinal kinematics.

Spine kinematics and anatomical shapes vary by region of the spine(cervical, thoracic, and lumbar), and a need exists for artificial discsaddressing specific regions of the spine, especially the unique geometryand kinematics of the cervical spine.

SUMMARY OF THE INVENTION

For the middle and lower regions of the cervical spine, the artificialdisc of the present invention adapts an articulating surface with aconcave-convex shape, also called a saddle shape. The artificial disc ofthe present invention is intended to fit substantially within theintervertebral space bound by adjacent vertebral bodies. A bone anchorfor fixation of an artificial disc to vertebra is also disclosed.

A first embodiment of the artificial disc of the present invention forthe cervical spine includes a disc body having an articulatingconcave-convex surface secured to a base plate, which may incorporate abone anchor. A second embodiment of the artificial disc of the presentinvention is comprised of an articulating concave-convex surface, boneanchor, and a disc body slidably attached to a base plate to permitadditional motion. A third embodiment of the artificial disc of thepresent invention includes an upper disc body and a lower disc bodycooperatively forming a saddle-joint. The upper disc body and lower discbody of the third embodiment are securely anchored to vertebral bodiesusing bone anchors. Further embodiments of the artificial disc of thepresent invention include an upper disc body and a lower disc bodycooperatively forming a saddle-joint, and bone anchors adapted with atension element to provide additional stability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a frontal view of a cervical spine motion segment comprisedof two vertebrae and an intervertebral disc.

FIG. 2A shows a cervical vertebra in a perspective view of the superiorendplate.

FIG. 2B shows an alternate perspective view of the cervical vertebrashown in FIG. 1A to emphasize the inferior endplate.

FIG. 3 shows a first preferred embodiment artificial disc of the presentinvention for the cervical spine with an articulating concave-convexsurface.

FIG. 4A, FIG. 4B, and FIG. 4C show the artificial disc depicted in FIG.3 in orthogonal views to include a front view, side view, and top view,respectively.

FIG. 5 shows the artificial disc depicted in FIG. 3 in an exploded viewto include adjacent vertebrae of the cervical spine.

FIG. 6 shows a midsagittal sectional view of the first preferredembodiment artificial disc of the present invention depicted in FIG. 3between adjacent cervical vertebrae.

FIG. 7 shows a midsagittal sectional view of a first preferredembodiment artificial disc of the present invention depicted in FIG. 3with a schematic of Instantaneous-Axis-of-Rotation (IAR) of a superiorvertebra in flexion.

FIG. 8 shows a second preferred embodiment artificial disc of thepresent invention with a disc body slidably attached to a base plate topermit rotational and translational sliding.

FIG. 9 shows a third preferred embodiment artificial disc of the presentinvention to include an upper disc body and a lower disc bodycooperatively forming a saddle-joint with concave-convex surfaces.

FIG. 10 shows a fourth preferred embodiment artificial disc of thepresent invention to include an upper disc body and a lower disc bodycooperatively forming a saddle-joint with concave-convex surfaces and atension cable.

FIG. 11 shows a fifth preferred embodiment artificial disc of thepresent invention to include an upper disc body and a lower disc bodycooperatively forming a saddle-joint with concave-convex surfaces and aflexible plate.

FIG. 12 shows a sixth preferred embodiment artificial disc of thepresent invention with an upper articulating concave-convex surface anda lower articulating surface.

DETAILED DESCRIPTION

Consistent with common medical nomenclature, superior is nearer the headin relation to a specific reference point, whereas, inferior is nearerthe feet in relation to a specific reference point. Anterior is forwardin relation to a specific reference point and posterior is rearward inrelation to a specific reference point. The midsagittal plane is animaginary plane dividing the body into a right side and left side. Afrontal plane is any imaginary vertical plane orthogonal to themidsagittal plane.

FIG. 1 shows a cervical spinal motion segment 1 characteristic of themiddle and lower cervical spine comprised of a superior vertebra 10, anintervertebral disc 7, and an inferior vertebra 10′. Superior vertebra10 is divided into two regions to include superior vertebral body 11 andsuperior posterior elements 17. Similarly, inferior vertebra 10′ isdivided into two regions to include inferior vertebral body 11′ andinferior posterior elements 17′. Although separated by theintervertebral disc 7 to permit motion, the superior vertebral body 11and the inferior vertebral body 11′ are anatomically interlocking due tothe reciprocal reception of the generally saddle-shaped inferiorendplate 23 of the superior vertebral body 11 with the superior endplate13′ of inferior vertebral body 11′. Although the spinal motion segmentsare not a synovial joint, the most closely related synovial joint to themiddle and lower cervical spine motion segments is the carpometacarpaljoint of the thumb, which is formed by articulating surfaces in the formof a saddle-joint. In contrast, the vertebral bodies of other regions ofthe spine (thoracic and lumbar) are not anatomically interlocking.

FIG. 2A and FIG. 2B show cervical vertebra 10 of FIG. 1 viewed atdifferent perspectives to highlight superior endplate 13 and inferiorendplate 23, respectively. Consistent with established medicalnomenclature, cervical vertebra 10 includes vertebral body 11, and thebony structures attached to vertebral body 11 are posterior elements 17.The superior endplate 13 is generally concave, or at least partiallyconcave, as indicated by the superior frontal surface line 14. Thesuperior midsagittal surface line 15 is substantially straight but maybe slightly concave or convex, or at least partially concave or convex.The inferior concave-convex endplate 23 is substantially convex in themedial-lateral direction and substantially concave in theanterior/posterior direction to form a saddle surface, as indicated byinferior convex surface line 24 and inferior concave surface line 25.

Primarily addressing the middle and lower cervical spine, the presentinvention uses a generally concave-convex articulating surface. As willbecome apparent in subsequent discussion, a concave-convex articulatingsurface is an essential element of the present invention, providinganatomical and biomechanical advantages. Referring now to FIG. 3, afirst preferred embodiment, artificial disc 100, is comprised of a discbody 130, base plate 138, and bone anchor 150. Concave-convexarticulating surface 133 is further revealed by considering two examplereference lines formed from convenient planer reference planes, concavesurface line 134 and midsagittal convex surface line 135. The curvatureof concave-convex articulating surface 133 may be formed of simple radiior variable radii, and may be further expressed mathematically, at leastin part, as a hyperbolic paraboloid, saddle-surface, or surface withnegative curvature As will become apparent in subsequent discussion, aconcave-convex articulating surface is an essential element of thepresent invention, providing anatomical and biomechanical advantages.Disc body 130 and base plate 138 are shown as separate components,although manufacture as a single component is also contemplated. Discbody 130 has a height appropriate for spacing of the vertebral bodiesand may be of various shapes and sizes in order to substantially fillthe surgically prepared space between vertebral bodies. Base plate 138may have planer surfaces and uniform thickness, as shown in FIG. 3,although variable thickness and curved surfaces are also contemplated.Bone anchor 150 and base plate 138 are adapted for fixation to bone.Bone anchor 150 is comprised of vertical anchor web 152, anchor body154, and anchor hole 156. As shown in FIG. 3, vertical anchor web 152has a vertical height greater than width. In addition, vertical anchorweb 152 may take other various forms, to include, but not be limited to,a web with variable thickness. Anchor body 154 is a protuberance with awidth substantially greater than the thickness of vertical anchor web152. As shown in FIG. 3, anchor body 154 is cylindrical, but othershapes are also contemplated, to include, but not limited to, a boxshape. A use of anchor hole 156 is to releasably attach a surgicalinstrument during insertion of artificial disc 100. FIG. 4A, FIG. 4B,and FIG. 4C show artificial disc 100 in orthogonal views to include afront view, side view, and top view, respectively

FIG. 5 shows the first preferred embodiment artificial disc 100 of thepresent invention depicted in FIG. 3 in an exploded perspective viewbetween superior cervical vertebra 110 and inferior cervical vertebra110′. Inferior vertebral body 111′ includes a surgically createdvertebral body key-hole 128′ intended to receive bone anchor 150. Boneanchor 150 provides secure interlocking fixation with inferior vertebralbody 111′ and requires a minimal amount of bone removal.

FIG. 6 shows a midsagittal sectional view of the first preferredembodiment artificial disc 100 depicted in FIG. 3 between superiorcervical vertebra 110 and inferior cervical vertebra 110′. Superiorvertebra 110 is comprised of superior vertebral body 111 and superiorposterior elements 117. Similarly, inferior vertebra 110′ is dividedinto two regions to include inferior vertebral body 111′ and inferiorposterior elements 117′. Concave-convex articulating surface 133 isintended to slide with respect to inferior concave-convex endplate 123provided the cartilaginous inferior concave-convex endplate 123 remainssubstantially intact following treatment of the patient's pathology. Thecontour of concave-convex articulating surface 133 is substantiallysimilar to the contour of inferior concave-convex endplate 123, althoughsome mismatch and variability is expected. It is expected that apatient's endplate can modify and generally adapt to the shape of animplant during healing. Bone anchor 150 is positioned within vertebralbody key-hole 128′. Base plate 138 may have additional features for bonyin-growth into inferior vertebral body 111′ through the use ofestablished porous materials or surface treatments. The anatomicallyaligned shape of artificial disc 100 results in a near complete fill ofthe intervertebral disc space; the resulting construct is substantiallyfree of large voids that are potentially susceptible to eventual tissueencroachment. Significant tissue encroachment into an artificial disccould potentially interfere with movement of an artificial disc.Finally, the anatomically aligned shape of artificial disc 100 isintended to fit within the intervertebral disc space with minimal boneremoval and endplate preparation during surgery, reducing surgical timeand preserving otherwise healthy tissue.

The anatomically aligned shape of the present invention also hasbiomechanical benefits associated with natural kinematics of the spine.To assist with the analysis of spine kinematics, Panjabi and Whiteestablished the Instantaneous-Axis-of-Rotation (IAR) for planer motionanalysis of vertebrae. Planar motion of vertebrae is fully described bythe position of the IAR and the angle of rotation about the IAR. The IARis an instantaneous measure and therefore may shift within a regionthrough a range-of-motion, such as, but not limited to,flexion/extension range-of-motion. The present inventors have discovereda relation between the shape of a vertebra's inferior endplate and thenatural motion of the same vertebra.

Flexion/extension is the most commonly considered degree-of-freedom whenevaluating cervical spine kinematics. FIG. 7 shows the first preferredembodiment, artificial disc 100 depicted in FIG. 3, in a midsagittalsectional view between superior vertebra 110 and inferior vertebra 110′.Superior vertebra 110 is shown in a neutral position with locator pointA1 and locator point B1 and in a flexed position with locator point A2and locator point B2. Locator points may be any two unique points on avertebra, and convenient anatomical landmarks are often used for thesereference points. The selected frame of reference is inferior vertebra110′, so the instantaneous-axis-of-rotation IAR is of superior vertebra110 with respect to inferior vertebrae 110′. Continuing to refer to FIG.7, instantaneous-axis-of-rotation IAR is established by determining theintersection of line LA and line LB, where line LA and line LB arebisected normal lines of translation vector A1A2 and translation vectorB1B2, respectively. Concave-convex articulating surface 133 has beenadapted to have a substantially similar shape to inferior concave-convexendplate 123. Continuing to refer to FIG. 7, inferior concave-convexendplate 123 sliding with respect to concave-convex articulating surface133 establishes the motion in flexion. The general region ofInstantaneous-Axis-of-Rotation IAR is within the posterior region ofinferior vertebral body 111′. Concerning the middle and lower cervicalspine, the Instantaneous-Axis-of-Rotation IAR in flexion/extension ofartificial disc 100, shown in FIG. 7, is consistent with recentscientific research demonstrating the IAR of a superior vertebra inflexion/extension generally lay within a posterior region of an inferiorvertebral body (DiAngelo et. al., proceedings of Cervical Spine ResearchSociety Meeting 2000). Further, the mathematical equations defining theshape of concave-convex articulating surface 133, generally in the formof a hyperbolic paraboloid, can be developed to substantially replicatethe complex natural motions of the spine, to include, but not limitedto, flexion/extension, lateral bending, and torsional motion.

Within the scope of the present invention, multiple components may beallowed to articulate to address multiple degrees-of-freedom associatedwith spinal motion, to include, but not limited to, torsional andtranslational degrees-of-freedom. Accordingly,

FIG. 8 shows a second preferred embodiment of the present invention,artificial disc 200 in an exploded perspective view. Artificial disc 200is comprised of a disc body 230, concave-convex articulating surface233, base plate 238, and bone anchor 250. Bone anchor 250 is comprisedof vertical web 252 and anchor body 254. Base plate protrusion 240inserts into disc body socket 236, while disc body articulating surface237 and base plate articulating surface 239 allow sliding rotation ofdisc body 230 about axis X-X 245. Alternately, disc body socket 236 maytake various shapes and sizes relative to base plate protrusion 240 toallow planer translation between disc body 230 and base plate 238. Forexample, disc body socket 236 may take the form of an elongated slot.Although, disc body articulating surface 237 and base plate articulatingsurface 239 are shown as planer surfaces, curved surfaces are alsocontemplated. A cylindrical shape for base plate protrusion 240 is shownin FIG. 8, however, other shapes are also envisioned within the scope ofthe present invention.

For a number of reasons, to include anatomical variation, the inferiorendplate of a superior vertebra may not be suitable as an articulatingsurface. During treatment of a cervical spine disorders, endplates areoften partially or completely removed in order to access and removeoffending soft tissue (e.g., extruded disc nucleus) or offending hardtissue (e.g., posterior bone spurs). A total joint artificial disc isoften warranted. Accordingly, FIG. 9 shows a third preferred embodimentof the present invention, artificial disc 300, comprised of lower discbody 330 and upper disc body 370. Lower disc body 330, attached to lowerbase plate 338, has a lower body concave-convex articulating surface333. Lower bone anchor 350 is comprised of lower vertical web 352 andlower anchor body 354. Upper disc body 370 is comprised of upper baseplate 378, upper body concave-convex articulating surface 373, and upperbone anchor 380. Upper bone anchor 380 is comprised of upper verticalweb 382 and upper anchor body 384. Lower anchor hole 356 and upperanchor hole 386 can be used to releasably attach a surgical instrumentduring implantation of artificial disc 300. Lower body concave-convexarticulating surface 333 is further defined by concave surface line 334and midsagittal convex surface line 335. Lower base plate 338 and lowerbone anchor 350 are adapted for fixation to an inferior vertebra.Similarly, upper bone anchor 380, comprised of upper vertical anchor web382 and upper anchor body 384 are adapted for body fixation to avertebra. The reciprocal reception of lower body concave-convexarticulating surface 333 with upper body concave-convex articulatingsurface 373 forms a saddle-joint. The complement of lower body concavesurface line 334 is upper body convex surface line 374, and thecomplement of lower body convex surface line 335 is upper body concavesurface line 375. Surface contours of lower body concave-convexarticulating surface 333 and upper body concave-convex articulatingsurface 373 may be dimensionally matched. Or, surface contours of lowerbody concave-convex articulating surface 333 and upper bodyconcave-convex articulating surface 373 may be cooperatively aligned,but dimensionally mismatched to give the saddle-joint additional freedom(“toggle”) or to establish selected regions of surface contact withinmanufacturing tolerances. A “gentle braking” occurs in torsion betweenthe lower disc body 330 and the upper disc body 370 as the lower bodyconcave-convex articulating surface 333 and upper body concave-convexarticulating surface 373 interact during a torsional motion away from aneutral position, reflecting a more natural resistance to torsionalloading. In addition, the surface geometry defining the articulatinginteraction of lower body concave-convex articulating surface 333 andupper body concave-convex articulating surface 373 may be developed tosubstantially replicate the complex natural motions of the spine, toinclude, but not limited to, flexion/extension, lateral bending, andtorsional motion.

Using an anterior approach to the cervical spine, the anteriorlongitudinal ligament is at least partially resected with associatedloss of stability, especially in extension. An artificial disc with atension element provides stability during extension. Referring now toFIG. 10, a fourth preferred embodiment is shown, artificial disc 400,comprised of lower disc body 430 and upper disc body 470. Lower discbody 430 is joined to lower base plate 438. Lower disc body 430 is alsocomprised of a lower body concave-convex articulating surface 433. Lowerbone anchor 450, comprised of lower vertical web 452 and lower anchorbody 454, is adapted for fixation to bone. Upper disc body 470 comprisedof upper base plate 478, upper body concave-convex articulating surface473 and upper bone anchor 480. Upper bone anchor 480 is comprised ofupper vertical web 482 and upper anchor body 484. One use of loweranchor hole 456 and upper anchor hole 486 is to releasably attach asurgical instrument during insertion of artificial disc 400. Asaddle-joint is formed by the reciprocal reception of lower bodyconcave-convex articulating surface 433 with upper body concave-convexarticulating surface 473. Tension cable 490, to include cable knot 491,is secured through the lower anchor eyelet 458 and the upper anchoreyelet 488. Tension cable 490 provides additional stability, restrictingmotion, especially during extreme extension.

FIG. 11 shows a fifth preferred embodiment, artificial disc 500,comprised of lower disc body 530 and upper disc body 570. Lower discbody 530 includes lower body concave-convex articulating surface 533.Lower disc body 530 is attached to lower base plate 538. Lower boneanchor 550, adapted for interlocking connection to bone, is comprised oflower vertical web 552 and lower anchor body 554. Upper disc body 570 iscomprised of upper base plate 578 and upper body concave-convexarticulating surface 573. Upper bone anchor 580 is comprised of uppervertical web 582 and upper anchor body 584. The reciprocal reception oflower body concave-convex articulating surface 533 with upper bodyconcave-convex articulating surface 573 forms a saddle-joint. Flexibleplate 590 is secured to lower anchor body 550 and upper anchor body 580by lower fastener 594′ and upper fastener 594, respectively. Flexibleplate 590 is intended to provide greater stability in extension. Toallow specified range-of-motion in extension and flexion where flexibleplate 590 is not substantially under load, lower fastener 594′ and upperfastener 594 are intended to slide with respect to lower plate slot 592′and upper plate slot 592, respectively.

Within the scope of the current invention, an artificial disc may havean upper articulating surface and a lower articulating surface forsliding interaction with vertebral bodies. FIG. 12 shows a sixthpreferred embodiment, artificial disc 600, comprised of disc body 630,upper articulating concave-convex surface 633 and lower articulatingsurface 673. Curvature of upper articulating concave-convex surface 633is further defined by concave surface line 634 and midsagittal convexsurface line 635. Lower surface line 674 is at least generally convexand lower midsagittal plane surface line 675 is generally straight, butmay also have curvature.

The present invention, to include, but not limited to the aforementionedembodiments, can be constructed of established orthopaedic materials.Established wear resistant materials for components with articulatingsurfaces include metals (e.g., stainless-steel, cobalt-chrome, andtitanium), plastics (e.g., Ultra-High-Molecular-Weight-Polyethylene),and ceramics (e.g., alumina and zironia). Non-articulating features ofan artificial disc of the present invention may have features ormaterial characteristics to facilitate rigid attachment to bone. Anartificial disc of the present invention may be adapted additionalfeatures known for attachment to bone including, but not limited to,spikes, screws, serration, and plate-like appendages generally exteriorto the intervertebral disc space. Components may be made, in part,constructed of substantially porous materials for bone in-growth, yethave smooth non-porous regions for articulating surfaces. Surfacetreatments commonly practiced, such as beaded-coatings, may also be usedfor attachment to bone through eventual bone in-growth. In addition,within the scope of the current invention, components may be givenflexibility through the use of geometry and materials to replicate thecushioning characteristics of the natural intervertebral disc.Additional components, such as, springs might be added to provideflexibility. In addition, portions of the patient's annulus may remainintact, such that the present invention augments a patient's existingintervertebral disc. Although the utility of the disclosed artificialdisc is best achieved in the cervical region of the spine, adaptationsfor the thoracic and lumbar regions of the spine are also within thescope and spirit of the present invention.

1. A prosthetic disc comprising: A disc body, having a disc body basesurface having a center point having a base surface central normalvector extending therefrom; and a disc body articulating surfaceopposite the disc body base surface, the articulating surface beingconcave with respect to a first disc body plane parallel to the baseplate central normal vector, and being convex with respect to a seconddisc body plane, parallel to the base plate central normal vector andorthogonal to the first disc body plane.
 2. The prosthetic disc of claim1 wherein the second disc body plane is the midsagittal plane.
 3. Theprosthetic disc of claim 1 wherein the curve of the articulating surfacein the first disc body plane is parabolic.
 4. The prosthetic disc ofclaim 1 wherein the curve of the articulating surface in the first discbody plane is hyperbolic.
 5. The prosthetic disc of claim 1 wherein thecurve of the articulating surface in the first disc body plane follows aradius.
 6. The prosthetic disc of claim 1 wherein the curve of thearticulating surface in the second disc body plane is parabolic.
 7. Theprosthetic disc of claim 1 wherein the curve of the articulating surfacein the second disc body plane is hyperbolic.
 8. A prosthetic disccomprising: A disc body, having a disc body base surface and a disc bodyarticulating surface opposite the disc body surface, wherein at least asubstantial region of said disc body articulating surface isconcave-convex.
 9. The prosthetic disc of claim 9 wherein the disc bodyarticulating surface is a hyperbolic paraboloid.
 10. The prosthetic discof claim 9 wherein the curve of the articulating surface in a first discbody plane is parabolic.
 11. The prosthetic disc of claim 9 wherein thecurve of the articulating surface in a first disc body plane ishyperbolic.
 12. The prosthetic disc of claim 9 wherein the curve of thearticulating surface in a first disc body plane follows a radius. 13.The prosthetic disc of claim 12 further comprising a base plate securedto the disc body base surface.
 14. The prosthetic disc of claim 13wherein the base plate comprises an anchor.
 15. A method of installing aprosthetic disc comprising the steps of: removing a portion of anintervertebral disc, thereby creating an intervertebral disc space, andplacing a prosthetic disc substantially within said intervertebral discspace; wherein the prosthetic disc comprises: a disc body, having a discbody base surface having a center point having a base surface centralnormal vector extending therefrom; and a disc body articulating surfaceopposite the disc body base plate surface, the articulating surfacebeing concave along a first disc body plane parallel to the base platecentral normal vector, and being convex along a second disc body plane,parallel to the base plate central normal vector and orthogonal to thefirst disc body plane.
 16. A prosthetic disc comprising: A first discbody, having a first disc body base surface having a first vectorextending normally therefrom, and a first disc body articulating surfaceopposite the first disc body base surface, the articulating surfacebeing concave along a first disc body first plane parallel to the firstvector, and being convex along a first disc body second plane, parallelto the first vector and orthogonal to the first disc body first plane,the outer regions of the first disc body articulating surface along andadjacent to the first disc body first plane defining a first disc bodyconcave region and the outer regions of the first disc body articulatingsurface along and adjacent to the first disc body second plane defininga first disc body convex region; a second disc body, having a seconddisc body base surface having a second vector extending normallytherefrom, and a second disc body articulating surface opposite thesecond disc body base surface, the articulating surface being concavealong a second disc body first plane parallel to the second vector, andbeing convex along a second disc body second plane, parallel to thesecond vector and orthogonal to the second disc body first plane, theouter regions of the second disc body articulating surface along andadjacent to the second disc body first plane defining a second disc bodyconcave region and the outer regions of the second disc bodyarticulating surface along and adjacent to the second disc body secondplane defining a second disc body convex region; the first disc bodyarticulating surface and the second disc body articulating surface beingdisposed in abutting relationship, and oriented such that at least aportion of the first disc body concave region is mated to at least aportion of the second disc body convex region or at least a portion ofthe first disc body convex region is mated to at least a portion of thesecond disc body concave region.
 17. An artificial disc suitable forplacement between adjacent vertebra comprising: a first disc body,having a first disc body base surface and a first disc body articulatingsurface opposite said first disc body surface, wherein at least asubstantial region of said first disc body articulating surface is ahyperbolic paraboloid; and a second disc body, having a second disc bodybase surface and a second disc body articulating surface opposite saidsecond disc body surface, wherein at least a substantial region of saidsecond disc body articulating surface is a hyperbolic paraboloid, andwherein said second disc body articulating surface is substantiallyreciprocal to first disc body articulating surface, wherein said firstdisc body articulating surface and said second disc body articulatingsurface being disposed in abutting relationship and cooperatively form asaddle-joint.
 18. An artificial disc suitable for placement betweenadjacent vertebra comprising: an upper body having an upper body basesurface and an upper body concave-convex articulating surface oppositesaid first disc body base surface; and a lower body, having a lower bodybase surface and a lower body concave-convex articulating surfaceopposite said lower body base surface; the upper body and lower bodyforming a saddle joint by the reciprocal reception of the lower bodyconcave-convex articulating surface with the upper body concave-convexarticulating surface.